Prior-art fiber-optic sensors and optical communication systems have used piezoelectric devices to impose a strain on optical fibers and thereby modulate signals transmitted by the fibers. Particularly E. F. Carome and K. P. Koo, IEEE Ultrasonics Sym., p. 710 (1980), report a construction in which a length of optical fiber is adhered to a length of oriented poly(vinylidene fluoride) (PVF.sub.2) piezoelectric film. The article further notes that since PVF.sub.2.
has the potential of being used as a protective jacket, . . . [the PVF.sub.2 ] might be formed as an integral part of an optical fiber.
Carome et al do not explain how PVF.sub.2 would be applied as a jacket on an optical fiber, and the characteristics of PVF.sub.2 present an obstacle to such use. PVF.sub.2 films as initially prepared have an alpha-phase crystalline structure that is not susceptible to piezoelectric properties. To give the films piezoelectric behavior, the films must be stretched, whereupon they assume a beta-phase crystalline structure that may be subsequently poled to a piezoelectric condition.
Theoretically an oriented poled film of PVF.sub.2 could be wrapped and bonded around an optical fiber, but that would be difficult to do, would require complex equipment, and generally would be economically unattractive. Coating PVF.sub.2 on the optical fibers would not be useful, because the optical fiber could not be subsequently stretched sufficiently to orient the PVF.sub.2 into a betaphase crystalline structure; nor is any method known for poling PVF.sub.2 coated on a small optical fiber (poling has generally been performed on flat films, usually with a metal electrode carried on the film).
A different teaching of a piezoelectric coating on an optical fiber is found in Davies et al, U.S. Pat. No. 4,002,896. After first describing constructions in which an optical fiber is either wrapped around a cylinder of piezoelectric material such as lead zirconate titanate or is bonded to a piezoelectric plate with adhesive, the patent suggests that a thin film of piezoelectric material could be vapor-deposited onto a fiber, or more specifically onto a metal layer previously coated over a plastic-cladded optical fiber.
The patent does not state what piezoelectric material is to be coated onto the fiber and there is no apparent useful material for the purpose. Lead zirconate titanate is the only piezoelectric material mentioned in the patent, but vapor-deposition of such a material would require high temperatures that would damage the plastic cladding on the fiber. If lead zirconate titanate were to be vapor-coated directly onto a glass-cladded optical fiber, the high-temperature processing would tend to damage the surface of the optical fibers and reduce the fiber strength. Also, previous applications of inorganic coatings on optical fibers have yielded fiber with poor optical properties because of the rough polycrystalline nature of the coating. Further, vapor-deposition of inorganic material would be a slow, cumbersome and difficult process. All in all, this patent leaves the art with a desire for a piezoelectric coated optical fiber but no apparent way to achieve it.